1. Field of the Invention
The present invention relates to a thin film deposition system, and more particularly to a pulsed laser deposition system utilizing a translational target and having an energetic beam directed in parallel to a substrate plane, thereby allowing material to be deposited over large areas of the substrate and characterized by a relatively short deposition distance and a small target.
Further, the present invention is directed to a technique permitting deposition of uniform film thicknesses over large, practically unlimited, surfaces of either planar or sophisticated contour configurations using a translational target assembly with an energetic beam directed towards the target surface in a direction parallel to the translational motion of the target.
More particularly, the present invention relates to a translational target assembly in thin film deposition systems wherein the target is brought close to the substrate, thereby changing the energetics of the plasma plume emerging from the target surface when the energetic beam impinges thereon, and further characterized by increased deposition rates. The overall system provides for a plume having an unchangeable or constant angle with respect to the distance of the plume from the substrate thereby providing exact reproductive characteristics of the deposition process for coating practically unlimited surfaces of the substrate.
2. Prior Art
Deposition of uniform films over large substrates is required for a variety of commercial applications, such as, for example in the design of sophisticated electronic devices. Specifically but not exclusively, such uniform films are necessary in providing superconducting delay lines with long delay times, or in fabricating multiple devices on a single substrate; to allow more efficient use of differently related process equipment needed to fabricate devices from deposited films. Generally, three techniques have been used to laser-deposit films over large areas. One of these three basic approaches to obtain large area films uses a rotating substrate and a large diameter counter-rotating target in conjunction with a rastered laser beam and large target-to-substrate distance. The technique has successfully been used to deposit Yba.sub.2 Cu.sub.3 O.sub.7-x (YBCO) over 75-mm-diameter substrates with good uniformity in both physical and electrical properties. A more recent version of a large area rastered system, modified to deposit films over 125-mm-diameter substrates, has a focused laser beam which is reflected from a programmable mirror held in a kinematic mount that allows the beam to be rastered over the entire diameter of the 77-mm-diameter ablation target. The rotating substrate is located 12.7 cm above the ablation target. The ablation target is offset with respect to the substrate. The advantages of this technique over the coaxial arrangement used in earlier raster systems includes the fact that a target with only about half the substrate diameter is needed to uniformly coat a given substrate size; and secondly, in the present configuration, the rastered laser beam crosses over the center of the ablation target, as the target rotates which results in the laser beam impinging at each target location from the opposite direction at a different time, reducing the growth of cones on the target surface, thereby decreasing the number of particulates that are ejected from the target surface.
In alternative large-area pulsed laser deposition (PLD) systems based on a fixed-position laser beam to deposit thin films, the laser beam was focused or imaged down close to the outer edge of a rotating ablation target. The simplest static beam approach, which was called off-axis PLD, was used to deposit films onto 25-mm-diameter substrates. In the off-axis approach, the center of the rotating substrate is offset a fixed amount, d, from the center of the ablation plume, as shown schematically in FIG. 1A. The offset distance, d, depends on the target-substrate spacing as well as the substrate diameter. If desired, a mask can be placed in front of the center portion of the substrate during the deposition run in order to impose a film's thickness in front. The off-axis process has been utilized to deposit YBCO films over 50-mm-diameter substrates, and more recently to deposit Bi.sub.4 Ti.sub.3 O.sub.12 thin films over 100-mm-diameter silicon substrates.
Another large-area PLD technique, based on a static position laser beam approach, utilizes both substrate rotation and computer-controlled substrate translation and has been used to deposit thallium-based superconducting oxide over 50-mm-diameter substrates. Rotational-translational PLD is shown schematically in FIG. 1B. In this case, the rotating substrate is translated back and forth in one direction with respect to the plume using a computer-controlled vacuum feed-through. Allowing the center of the ablation plume to impinge close to the outer edge of the rotating substrate for a longer period of time, the properties of the deposited film exhibit good uniformity. This process has been further refined to deposit thallium-based superconducting compound over three 50-mm-diameter substrates simultaneously.
Simple and fast deposition technique for large-area high-temperature superconducting (HTSC) thin films which are necessary for the realization of HTSC devices, for example, in microwave applications, was described in the article to M. Lorenz et al, "Large-area double-side pulsed laser deposition of YBa.sub.2 Cu.sub.3 O.sub.7-x thin films on 3-inch sapphire wafers", Applied Physics Letters, 68 (23), Jun. 3, 1996, pg. 3332-3334. The technique uses the off-axis PLD technique for deposition of YBCO and gold, and a rotational-translational PLD approach is applied for CeO.sub.2 buffer layers. In the off-axis approach, the center of the rotating substrate is offset a fixed amount from the center of the ablation film. An offset of about 30 mm is used with a target-substrate distance of 90 mm. With the rotational-translational approach, in addition, a controlled substrate translation during deposition is utilized in order to improve homogeneity of film properties.
Another PLD system, which has been proven to be viable in producing large-area YBCO films, was presented in the article by K. H. Wu et al, "Preparation of large-area and investigation of initial film growth of YBa.sub.2 Cu.sub.3 O.sub.7 by scanning pulsed laser deposition", Applied Physics Letters, 69 (3), Jul. 15, 1996, pg. 421-423. In this system, a planar reflector and a concave reflector were used to guide the laser beam to a 50-mm-diameter YBCO target situated in a vacuum chamber. As best shown in FIG. 2, both the planar reflector and the target were rotated by dc motors with a small angle between the motor axis and the reflector (or target) axis, so that the reflected laser beams scanned a circle prior to impinging on the concave reflector. The focused laser beam then scanned a circle of about 25 mm in diameter on the rotating target and generated an extended plume with a diameter of 40 mm.
All the above discussed PLD systems for thin films deposition proved to be viable for deposition of films with desired uniformity of thickness and electrical properties, and demonstrated satisfactory growth rates over large-area substrates that are competitive with other physical deposition processes. However, there are still several issues that need to be addressed and phenomena that should be understood before PLD becomes a viable tool of film deposition on large areas. For instance, since the laser beam impinges to the target surface at a predetermined angle, in order to provide deposition on a large area of the substrate, the target-substrate distance must be larger. Increasing this distance to 10 cm or more, disadvantageously, will have an impact on the deposition rate as well as optimum gas pressure used to grow a given material with a desired set of properties. In lieu of the above said, it will be highly desirable to provide a PLD system for thin film deposition on a large area which would allow for providing depositions on large substrates while simultaneously maintaining a minimum target-substrate distance.
A prior art system has been developed which lowers the large substrate-target distances for depositing material on large substrates. This system which has been designed for pulsed laser deposition of homogeneous YB.sub.2 Cu.sub.3 O.sub.7-x films on substrates as large as 7.times.20 cm.sup.2 has been described in the article by B. Schey et al, "Pulsed Laser Deposition of YBCO Thin Films on 7.times.20 cm.sup.2 Substrates". As best shown in FIG. 3, as opposed to the conventional PLD arrangements discussed above, where the light of an Excimer laser is focused to a point on the target, the setup shown with an 8 cm focused line on a cylindrical target is used. A scanning of the substrate perpendicular to the focused line with an amplitude of 32 cm, leads to a homogeneous deposition of 7.times.20 cm.sup.2 areas. The heater used in the setup consists of a ceramic box with dimensions of about 18 cm.times.65 cm.times.6 cm. Conventional heating wires are integrated in the ceramic top and bottom walls to provide a homogeneous temperature inside the ceramic box. The substrates are scanned inside this box and can be coated through a window (10 cm.times.5 cm) by the plasma. As it is understood, the substrate translates in scanning direction relative to the window in the ceramic top of the ceramic box. However, such prior art does not suggest any mechanism which provides translational motion of the targets along the scanning direction or any rotation of the target which may be interrelated with the translational motion of the targets.
The afore-discussed prior art system with the laser beam parallel to the scanning direction of the substrate is not limited to large target-substrate distances and is adaptable for coating large substrate areas. The present invention, however, taking advantage of the laser beam directed in parallel to the substrate surface, introduces a new and unique approach to PLD technique, which allows coating practically unlimited surfaces with uniform thin films by means of employing translational motion of the target relative to the substance in parallel to the laser beam, and both in parallel to the surface of the substrate.